In a watch, a striking mechanism may be provided to generate a sound or piece of music. To achieve this, the gong of the striking watch or the vibration plate of the musical watch are generally arranged inside the watch case. Thus, the vibrations of the gong or the vibration plate strips are transmitted to the various external parts of the watch. These external parts are, for example, the case middle, the bezel, the crystal and the back cover of the watch case. These large external parts start to radiate sound into the air under the effect of the transmitted vibrations. When a sound is produced either by a gong struck by a hammer, or by the vibration of one or more strips of the vibration plate, these external parts are capable of radiating the produced sound into the air.
One or more membranes arranged inside the watch case may be used as additional external components or elements. These membranes are configured to improve the acoustic level of the sound or music produced. Other external watch parts may also be adapted to produce good sound radiation, notably at low frequencies. However, it is difficult to envisage optimising the acoustic radiation of the back cover of the watch case, which is an external part that usually remains virtually inactive acoustically.
It is to be noted that, when the watch is worn on a user's wrist, the vibration of the lower back surface of the watch case is damped on contact with the wrist. In such case, the only solution for creating a sound or piece of music leaving the back side of the watch case consists in using lateral openings. Radiation through the lateral openings in the back cover remains low however, since several parts surrounding said openings are rigid. Further, if the back cover is provided with a crystal allowing the movement to be viewed from below, dirt can enter through the openings in the back cover and thus be deposited on the lower surface of the back crystal. Such a back cover provided with a crystal also suffers from inadequate sealing. These are drawbacks.
In a musical or striking watch, acoustic efficiency, based on the complex vibroacoustic transduction of the external parts, is generally low. In order to improve and increase the acoustic level perceived by the user of the striking or musical watch, the material, geometry and boundary conditions of the external parts must be taken into account. The configurations of these external parts are also dependent upon the aesthetic appearance of the watch and operating stresses, which may limit adaptation possibilities.
The frequency content of the sound from a striking or musical watch must be rich in a frequency range between 1 kHz and 6 kHz. Conventional external parts do not allow efficient radiation in this frequency range. As described above, in order to further improve the vibroacoustic efficiency of the striking mechanism, one or more membranes may be placed inside the watch case. The membranes can easily be sized and configured for efficient radiation of the note or notes produced in the watch case. The frequencies of the notes produced must be close to the natural vibration modes of the membranes in order for them to resonate.
Constraints relating to the arrangement of acoustic membranes are generally at variance with the rules of mechanical design for ensuring impermeable sealing, and the mechanical resistance of the watch to shocks and high external pressures. The back cover of the striking or musical watch, which is normally pierced with openings, also requires the connection between the membrane or membranes and the rest of the movement to remain sealed. Further, a membrane having a too low level of stiffness cannot ensure sufficient resistance to external pressure without risking damage to the movement, which is a drawback.